It sounds like a trip to the vet is needed...or at least a phone call to see what they think of a descripton of her condition. My Belladonna behaved that way from an infected bite in the base of her tail... because of her lush long fur it was virtually invisible. Cats get abcesses very easily from minor wounds gotten in play with other cats... so it could be that, or it could be constipation or some othr injury... I once had a cat that slipped a disc!

A drastic change in behavior and trouble moving in a normal way (sitting) or being protective of a body part means something is not ok and needs to be checked out.